It is often desirable in biomedical applications to analyze one or more physical properties and/or constituents of small volume samples of a patient's body fluid. For example, samples of a patient's whole blood are often analyzed and/or monitored to provide information regarding blood analytes such as pH, pCO2, pO2, K+, Na+, Ca++, Cl−, hematocrit and the like. Information derived from analytes in a blood sample is compared to normal physiological function and homeostasis profiles and may be used by a physician for diagnostic purposes, patient monitoring and/or for the control of life support systems, for example.
Systems which employ electrochemical electrodes for detecting constituents of a body fluid are well known in the art and are described, for example, in U.S. Pat. Nos. 3,658,478; 5,387,329; 5,338,435; 4,734,184; 4,361,539 and 5,200,051, the entire contents of which are hereby expressly incorporated by reference. Typically, systems which provide blood chemistry analysis are stand-alone machines or are adapted to be connected to an extracorporeal shunt or an ex vivo blood source, e.g., a heart/lung machine used to sustain a patient during surgery. Small test samples of flowing live ex vivo blood may be diverted off-line in real-time from either the venous or arterial flow lines of the heart/lung machine directly into a chamber and exposed to a bank of microelectrodes containing sensors which generate electrical signals proportional to, or indicative of, chemical characteristics of the real time flowing blood sample.
There have been efforts to miniaturize the sensors themselves and to fabricate them by techniques recently made available by developments in integrated circuit technology. In this regard, integrated circuit technology allows sensors to be fabricated in a planar form, whereby thin layers of materials are applied successively to a base dielectric substrate using thick-film and/or thin film techniques. The manufacture of planar sensors can be significantly automated to allow production in quantity and at lower cost. Planar sensors can be made smaller and configured more closely together, reducing the fluid sample volume requirements.
Though various miniature electrochemical sensors and Microsystems using such sensors have been developed, the unavailability of a durable, stable and reliable miniature reference electrode has restricted the use of miniature electrochemical sensors for industrial and biomedical applications. The stability of a reference electrode in potentiometry is very important for the reliability and accuracy of the measurements. A potential error of only 1 mV causes about a 0.02 pH error in pH measurements and 4% or 8% concentration error, respectively, for mono-and divalent ion measurements using ion-selective electrodes. Nevertheless, miniaturization has been largely limited to the development of working and indicator electrodes and, in many cases, commercial macro reference electrodes have been used in the absence of a reliable miniature reference electrode.
There have been several attempts to make miniature reference electrodes, as separate, independent units, as subcomponents in a multi-sensor structure, or integrated with various miniature ion-selective sensors. Typically, such systems use a thin film Ag/AgCl electrode. The thin film Ag/AgCl electrode is based on a very high exchange-current density reaction, i.e., at low current densities the electrode is not polarized and the potential at the electrode/ electrolyte interface is only a function of Cl− activity. However, several factors can limit the durability of the Ag/AgCl element. Conventional macro liquid-junction Ag/AgCl electrodes consist of a reservoir with a high activity of KCl, saturated with AgCl. Since AgCl shows some degree of solubility in high Cl− activities, a thin film of AgCl can ultimately dissolve depending on the temperature of the bridge electrolyte solution. In addition, the standard liquid junction miniature electrodes do not provide an adequate contact surface or liquid junction between a sample and the reference bridge electrolyte solution that completes the circuit for the reference electrode. A need therefore exists for an improved miniature reference electrode.